In printing processes such as electrophotographic printing, many toner images are produced in many printing units or repetitively, in one printing unit in order to produce a print image on a print material. In general, these toner images contain the colors cyan (C), magenta (M), yellow (Y), and black (K). To produce these toner images, first a latent electrostatic image is formed on an imaging medium. The imaging medium can be an imaging cylinder or a corresponding imaging band that, in each case, has a photoconductive surface layer.
The imaging cylinder is exposed, for example, by a row or a field of laser diodes or LEDS, a latent image is produced in this manner. Due to the light, previously charged regions of the surface of the imaging cylinder are discharged. The imaging cylinder then passes a development unit where toner is transferred via an inking unit to the surface of the imaging cylinder, and is held there through electrostatic forces. Dry toner or liquid toner can be used that in each case, has charged particles.
A voltage is applied to the area between the surfaces of the development unit and the imaging cylinder. Depending on the various potentials of the surfaces and on the function of the charge of the toner particles used, they remain adherent to the unexposed areas (charged area development, CAD) or to the exposed areas (discharged area development, DAD) of the imaging cylinder. Here, particularly, the charge sign of the toner particles are significant. For further discussion, see, for example, “Electrophotography and Development Physics”, in the second revised edition by L. B. Schein, 1996 that appeared as a reprint from Laplacian Press, pp. 32 ff.
The toner can be transferred directly from the imaging cylinder to print material with the aid of electrostatic forces. It is also common to use an additional transfer medium. Thus, the toner is transferred first from the imaging cylinder to the transfer medium, and can then be transferred from the transfer medium to the print material. The transfer medium can be, for example, a rubber blanket cylinder or a transfer belt.
When using transfer belts specifically, another possibility arises namely, that the toner images of the individual printing units are first transferred atop one another on the transfer belt and are then transferred in one step, from the transfer belt to the print material.
After the various toner images are applied on a print material atop one another, they are fused in a fusing apparatus onto the print material. This can be accomplished through the influence of pressure and heat on the print material. The print material is transported for this purpose, with a transport mechanism, such as a conveyor belt or transport rollers with grippers through the printing press. A fusing apparatus for example, has a fusing roller and a counter-pressure cylinder. Both the fusing roller and the counter-pressure cylinder can be heated. The print material can then be transported with the toner through the nip that is formed by the fusing roller and the counter-pressure cylinder. The toner is then fused on the print material through heat and pressure.
One problem that can arise in using such a fusing procedure is known as an “offset” of the toner. In this case, toner can adhere to the fusing roller and possibly to the counter-pressure cylinder and thus, soil these components. In order to prevent an offset, silicone oil and/or other oil-bearing substances are applied as a separating medium to the surface of the fusing roller and perhaps to the counter-pressure cylinder. However, the usage of such oil-bearing substances has also proven to be problematic.
In a duplex printing process, a print material is printed on both sides. There are various alternative methods of executing this process. For each side of the print material, independent printing units can be used. Then, with the same printing process used to print the first side, the second side is also printed and the toner images on both print material sides can be simultaneously fused on the print material.
In an alternative process, the first print material side is printed. The print material is then rotated in a rotating device before the second side is printing in the same printing units. Only after this second passing of the print material passes through the printing units of the printing press, this second time, is the print material fed through a fusing apparatus. The toner images on both sides of the print material are then fused simultaneously fused by the fusing apparatus on the print material.
One problem with this alternative process is that the unfused toner images abut onto the transport mechanism during the second passing of the print material through the printing units. The toner images may endure smearing before they are definitively fused.
In a third preferred duplex printing process, the first print image produced by the printing units on the first print material side, is fused before the second side of the print material is printed. To achieve this purpose, a second independent printing press can be used, or, preferably, the print material can be rotated, by a rotating device, and the second side can be printed with the same printing units previously used to print the first side. The print image produced in this manner on the second print material side, is then finally fused by the same fusing apparatus on the print material, as was the print image on the first print material side.
Particularly, in multicolor printing presses, it is not desirable, in terms of cost and space, to install a second set of identical printing units within the printing press. Thus, the first duplex printing process presented is frequently rejected. In order to avoid the smearing of toner on the print material that can occur in a second run through the printing press, the third duplex printing process is frequently used.
As described above, to avoid an offset of toner within the fusing apparatus, oil-bearing substances, particularly silicone oil, are used as a separating medium. This separating medium should, if possible, form a closed layer on the surface of the fusing roller, or alternatively on the counter-pressure cylinder. In other words, sufficient separating medium must be applied. Since the separating medium comes into contact with the surface of the print material during the fusing procedure oil-bearing substances adhering to the surface of the fused print material cannot be avoided.
These oil-bearing substances are found on the side of the print material that abuts, onto the transport mechanism in a duplex printing process, during the second pass through the printing units. Accordingly, the oil-bearing substances can also get onto the surface of this transport mechanism and therethrough into the individual printing units.
These oil-bearing substances can unfavorably disrupt the transfer of toner between the individual areas: within an inking unit, from the development unit, or alternatively the inking unit onto the imaging cylinder, or from the imaging cylinder onto a transfer medium such as a rubber blanket cylinder, and finally, onto the paper. This impairment of the toner transfer can lead to smearing or streaking formations on the print image. The varying toner density can also negatively affect a print material.
More silicone oil is applied on the fusing roller at the beginning of a printing process than at a later time point. If a uniform layer of silicone oil is formed on the fusing roller initially, less oil needs to be applied onto the fusing roller. Thus, the toner transfer at the beginning of a printing press is more intensive, but not as impaired as at a later point in time. Accordingly, one must either accept greater quality fluctuations within a printing process or endure more waste paper.
If oil-bearing substances adhere to the imaging cylinder, it can disrupt the electrophotographic process, and lead to uncontrollable variances in the print image produced.